Predicting the remaining capacity and reserve time of a battery is important to the proper management of battery plants and of batteries remotely located. The fundamental method of prediction, of the prior art, is based on the Peukert equation: EQU t=aI.sup.b ( 1)
where t is the reserve time to a given end voltage, I is the discharge current and "a" and "b" are empirically determined parameters. The remaining reserve time during discharge is obtained by subtracting the actual time of discharge from the value "t" given by the equation. The only real time data used in this approach is the discharge current I, and the parameters "a" and "b" must be experimentally determined by extensive testing, data acquisition and parametric analysis. Once determined, these values are fixed and do not adapt to changing conditions and are not responsive to changing load requirements.
An attempt to be more responsive to changes in battery behavior during discharge is disclosed in the patent application Ser. No. 08/013272, filed Feb. 4, 1993, submitted by D. Levine et al which utilizes matrices of predetermined parameters that correlate the slope of the voltage-versus-discharge time at various discharge currents, battery voltages during discharge and end voltages. The use of voltage-versus-time slopes for prediction allows the method to be highly adaptable to changes in battery behavior during discharge. This method requires extensive testing to derive the data to populate the matrices.
Another approach, disclosed by R. Biagetti and A. Pesco in U.S. Pat. No. 4,952,862, operates by measuring the difference between battery voltage during discharges and the battery plateau voltage, EQU V.sub.battery -V.sub.p ( 2)
During discharge this difference is plotted against a ratio of discharged capacity to the total discharge capacity available: EQU Q.sub.removed /Q.sub.to-end-voltage ( 3)
This plot, created from measured data, is a single curve having an exponential and a linear region. The curve is used to determine remaining capacity and reserve time from the measured discharged capacity Q.sub.removed and the plateau voltage V.sub.p. As in the above described method, extensive prior testing and data analysis of the particular battery being monitored is required, and the method does not account for aging of the battery since of the plateau voltage is a predetermined fixed value.
Another approach to determining the reserve time of a discharging battery, disclosed in U.S. Pat. No. 4,876,513, takes advantage of the fact that when battery voltages (corrected for internal resistance) are plotted versus a ratio of ampere hours remaining to ampere hours available to a certain discharge voltage all discharge curves fall on a single curve. The battery voltages are calculated using a battery internal resistance that is measured periodically during discharge.
None of the existing methods for evaluating the state of a discharging battery works accurately at all temperatures, requires only a minimal number of parameters and is independent of the battery type being monitored.